[MPS] Add support for Custom Kernels (#100661)
- This change introduces these APIs to enable developing custom kernels on the MPS Stream:
`torch::mps::get_command_buffer()`
`torch::mps::get_dispatch_queue()`
`torch::mps::commit()`
- Add ObjC test case
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100661
Approved by: https://github.com/kulinseth, https://github.com/malfet
